References That Survive an Edit
When you fillet an edge and then change the extrude that made it, the fillet has to find that edge again on the rebuild. Most of the time it does, without telling you. When it cannot, the step turns yellow or red, and how you made the edit decides which.
What a pick stores
Picking a face does not store a position. It stores an identity: which step first created that face, a slot name for it such as top or side_3, the chain of steps the face came through afterwards, and a rough geometric fingerprint of where it is, which way it points, and how big it is.
The fingerprint is the last resort, not the first. That ordering is what makes the model editable at all: a face that moves 40 mm because you doubled a plate is still the same face, and identity says so where geometry would not.
How it looks for it again
On every rebuild, each pick is resolved in order. The exact element first, then the same element traced through the steps that made it, then a unique slot name, then the ancestry chain. Only when all of those fail does the app fall back to measuring geometry and choosing the closest candidate. If that fails too, the step errors.
That gives three outcomes you can see:
- Nothing on screen. Matched by identity. This is the normal case and it is silent.
- A yellow warning banner in the step's panel. Matched by geometry, and flagged as uncertain because identity did not settle it.
- A red banner. Not matched. The step has failed.
A whole selection is resolved together and injectively: if you picked four edges you get four distinct edges back, never the same edge twice.
How far geometry can move
When it comes to the geometric fallback, these are the windows a candidate has to fall inside.
| Element | Must still be |
|---|---|
| Face | Facing the same way, near enough exactly. In the same plane to within 0.1 mm. Centre within 2 mm. Area between half and double |
| Edge | Ends and midpoint within 2 mm. Length between half and double |
| Circular edge | Centre within 2 mm. Radius between half and double |
| Vertex | Within 2 mm |
The direction test is signed, so a face that has flipped never matches, however perfectly it lines up. The app counts anything that has moved less than 0.05 mm as not having moved at all. The ancestry it follows caps at eight steps, so an element that has passed through more operations than that has less history to fall back on.
What this means when you edit
The number to remember is 2 mm. A face that moves further than that in a single edit is outside every window, so if identity has already failed, the reference is lost even though the face is plainly still there.
The good news is that most print-driven editing is well inside it. Opening a hole by 0.2 mm for a better fit, taking a wall from 1.2 mm to 1.6 mm, adding a tenth to a clearance: none of that puts a reference at risk. Taking a plate from 20 mm to 35 mm in one edit does.
Four habits follow.
Make big changes in a few steps. When a downstream fillet or shell depends on a face, change the driving dimension in two or three moves rather than one, and let the rebuild re-anchor each time.
Watch for changes of scale. Doubling or halving a face's area leaves the window as certainly as moving it does, so widening a slot from 4 mm to 10 mm is a bigger risk than it looks.
Never flip a face you have picked on. Switching an extrude from Add to Subtract, mirroring a profile, reversing a direction: any of these turns a face round, and a turned face is a lost reference.
Do not renumber a row. Slot names for unnamed elements are ordinal. Adding a hole in the middle of a row of six shifts the names of everything after it, and a fillet that pointed at the fourth hole may come back pointing at the third.
And the habit that avoids the problem outright: anchor to reference planes, axes and points rather than to faces of the model, because a reference is not remade by every step upstream of it. Using References covers when that is worth the extra step.
The failure that does not announce itself
A geometric fallback can be wrong without being an error. It takes the best candidate inside the windows, and on a symmetric part there may be more than one plausible answer. It is marked uncertain and the yellow banner in the panel is the only signal you get.
So a yellow banner is worth opening. Look at what is actually highlighted in the viewport, and if it is not what you meant, clear the pick and pick again. The step is not going to complain, and neither is the export.
The same thinking is behind the confirmation you get when you turn a sketch's Bounded to face toggle on. Clipping a sketch to a face changes which closed regions it has and what they are called, so anything that used a region may come back holding a different one.
When it goes wrong
A red banner reading "A referenced face/edge/vertex no longer exists". The element is gone, usually because the step that made it was deleted or changed enough to remove it. Open the failed step, clear the pick, and pick the replacement.
A red banner reading "A referenced feature is suppressed". Something the step depends on has been disabled. Right-click that step's pill and choose Enable.
A yellow banner. Matched by geometry. Check what was matched before you carry on.
Several steps red after one small edit. Look for the earliest red pill rather than the loudest one. The rest are consequences of it, and fixing the first often clears the lot. When a Step Goes Red has the reading order.
See also
- Using References: anchoring to a plane or axis instead of a face
- When a Step Goes Red: the families of rebuild error and what to do about each
- Editing a Part You Have Built: the edit loop these references have to survive
- Working with Several Bodies: why leaving the boolean until last keeps picks stable